1. Field of the Invention
The present invention relates generally to semiconductor manufacturing and, more particularly, to the monitoring and control of lithographic process conditions used in microelectronics manufacturing.
2. Description of Related Art
The lithographic process allows for a mask or reticle pattern to be transferred via spatially modulated light (the aerial image) to a photoresist (hereinafter, also referred to interchangeably as resist) layer or film on a substrate. Those segments of the absorbed aerial image, whose energy (so-called actinic energy) exceeds a threshold energy of chemical bonds in the photoactive component (PAC) of the photoresist material, create a latent image in the resist. In some resist systems the latent image is formed directly by the PAC; in others (so-called acid catalyzed photoresists), the photo-chemical interaction first generates acids which react with other photoresist components during a post-exposure bake to form the latent image. In either case, the latent image marks the volume of resist material that either is removed during the development process (in the case of positive photoresist) or remains after development (in the case of negative photoresist) to create a three-dimensional pattern in the resist film. In subsequent etch processing, the resulting resist film pattern is used to transfer the patterned openings in the resist to form an etched pattern in the underlying substrate. It is crucial to be able to monitor the fidelity of the patterns formed by both the photolithographic process and etch process, and then to control or adjust those processes to correct any deficiencies. Thus, the manufacturing process includes the use of a variety of metrology tools to measure and monitor the characteristics of the patterns formed on the wafer. The information gathered by these metrology tools may be used to adjust both lithographic and etch processing conditions to ensure that production specifications are met. Control of a lithographic imaging process requires the optimization of exposure dose and focus conditions in lithographic processing of product substrates or wafers.
Lithographic systems consist of imaging tools that expose patterns and processing tools that coat, bake and develop the substrates. The dose setting on the imaging tool determines the average energy in the aerial image. Optimum dose produces energy equal to the resist threshold at the desired locations on the pattern. The focus setting on the imaging tool determines the average spatial modulation in the aerial image. Optimum focus produces the maximum modulation in the image. The settings of many other imaging and processing tool parameters determine the effective dose and defocus (deviation from optimum focus) that form the latent image in the resist film. For advanced imaging tools, such as step-and-scan exposure systems, imaging parameters that determine the effective dose and defocus include the dose setting, slit uniformity, mask-to-wafer scan synchronization, source wavelength, focus setting, across-slit tilt, across-scan tilt, chuck flatness, etc. For advanced processing tools, processing parameters that determine the effective dose and defocus include the coat thickness and uniformity, the post-expose bake time, temperature and uniformity, the develop time, rate and uniformity, wafer flatness, topography, etc. Typically, the different imaging and process parameter sources of variation can be distinguished by the spatial signature of the effective dose and defocus variation they cause.
Variation in both imaging and process parameters cause variations in the spatial distributions of effective dose and defocus in the resist film that, in turn, cause variations in the dimensions of the printed patterns. Because of these variations, patterns developed by lithographic processes must be continually monitored or measured to determine if the dimensions of the patterns are within acceptable range.
Tight process control is a continuing challenge for advanced microlithography processes, such as those used to manufacture state-of-the-art integrated circuits. In normal practice, process control marks printed in the kerf of the chip are measured for compliance to a line width control specification. If the measurement is found to be outside control limits, then corrective actions are taken. Unfortunately, an out-of-spec line width measurement does not clearly point towards a root cause, and this complicates the task of taking the proper corrective action. Many line width deviations are caused by either a focus deviation or an exposure variation, and U.S. Pat. No. 5,965,309 discloses analysis methods which can separately determine these two key quantities.
Other prior art methods have been used to determine variations in exposure dose and focus during lithographic processing. U.S. Pat. No. 5,300,786 discloses a focus monitor test pattern which can accurately determine the sign and magnitude of the defocus value, but requires the use of a phase shifting mask, which may not be compatible with most products. An exposure monitor intended to be particularly sensitive to exposure dose via a mask edge, and which would transition from dark to bright by a series of gradually changing sub-resolution lines, was disclosed in A. Starikov, SPIE Vol. 1261, pp. 3 15-324 (1990). Such Starikov exposure monitor patterns are extremely difficult to build because of their very small dimensions, and are not periodic, and consequently cannot be measured using scatterometry.
U.S. Pat. No. 7,042,551 discloses use of isolated gratings where the size of the individual elements is small with respect to the pitch of the grating. The process window is small compared to the features on the chip, and there is a limit to how isolated one can make these gratings. Other methods are described in U.S. Pat. Nos. 5,953,128; 5,976,740; 6,004,706; 6,027,842 and 6,128,089.